This invention relates to ink jet printer inks, and in particular to such inks comprising a dispersion of a solid pigment dye in a diluent.
In ink jet printing, a fluid ink is forced under pressure, and sometimes at elevated temperature, through a very small nozzle in a printing head. In one kind of printer, known as a xe2x80x9ccontinuousxe2x80x9d printer, ink droplets which are produced continuously are passed through a charging area where individual droplets receive an electrical charge in response to a signal and are directed towards a substrate to be printed. The droplets then pass through an electrical field causing them to be deflected by an amount which is dependent on the intensity of the charge and the field. Droplets not required to form print on the substrate are directed to a by-pass gutter. Inks for use in such printers need to be conductive.
In another kind of printer, known as a xe2x80x9cdrop-on-demandxe2x80x9d (DOD) printer, the ink droplets are expelled from the nozzle of a print head only when required during the printing process. Drop-on-demand printers can use an electrostatically accelerated ink jet or droplet sequences ejected by pressure impulse actuation. In the latter kind of DOD printer, each drop of ink is individually ejected from a nozzle by means of pressure pulses induced e.g. by use of a piezo-electric actuator acting on the ink in the channel supplying the nozzle or by generation of a vapour bubble in response to a thermal pulse.
For an ink in the form of a dispersion to be suitable for use in modern ink jet printers, especially DOD printers, it must meet a number of essential criteria. It must be stable so that phase separation or sedimentation does not occur while the ink is in store or is in the head of the printer; it must have a sufficiently low viscosity that the energy requirements for firing it from a print head are acceptable; it must be capable of being fired from the printhead consistently in the form of single droplets of uniform size and it must dry quickly on the substrate to produce a printed dot of acceptable colour density and a well-defined regular outline. To meet these criteria requires the ink to have a viscosity, surface tension and vaporisation rate within certain well defined limits. It is also important that the ink does not wet the surface of the ink jet nozzle since this leads to the need to clean the nozzle frequently, thus interfering with long print runs. A measure of this property is the receding meniscus velocity (RMV) of the ink.
These properties of viscosity, surface tension, vaporisation rate and RMV are all a function of one or both of the diluent and the dispersant employed in the ink composition.
Dispersion inks tend to fall into two groups, those wherein the diluent is aqueous and those wherein it is essentially non-aqueous. Aqueous inks are essentially odourless but have certain shortcomings; in particular their drying rate is not as fast as most non-aqueous inks and the print tends not to be water fast. Non-aqueous inks are generally quicker drying and give water-fast print; however, the print from non-aqueous ink compositions developed hitherto has tended to exhibit an odour which is considered unpleasant by some users. There has therefore been a demand for an odourless or substantially odourless non-aqueous ink composition which retains the properties required of an ink jet printer ink, in particular, the necessary viscosity, surface tension, vaporisation rate, non-wetting characteristics and stability; specifically:
a viscosity of not greater than 35 mPa.s measured at 25xc2x0 C. using a Bohlin CS Rheometer with a CP 4/40 measuring system; preferably no greater than 15 mPa.s, more preferably no greater than 12 mPa.s;
a surface tension in the range 22 to 36 mN.m at 25xc2x0 C., more preferably 24 to 32;
a boiling point greater than 200xc2x0 C.;
an RMV of at least 1.0 mm.secxe2x88x921, measured as described in WO97/15633, more preferably at least 10 mm.secxe2x88x921; and
a stability such that the viscosity of the ink has not changed by more than 50%, preferably not more than 20% and more preferably not more than 10%, after 6 weeks ageing at 70xc2x0 C. and there is no clear indication of phase separation of the said aged ink as evidenced by the observation of ink particles retained on a spatula which has been inserted in a vial containing the aged ink, rubbed along the bottom of the vial, and then withdrawn for examination.
WO95/01404 describes an ink jet ink comprising a liquid having an electrical resistance of at least 109 ohm cm, insoluble marking particles and a particle charging agent. The ink is designed for use in the type of inkjet printer described in WO93/11866. This is a printer having a triangular shaped plate over which the ink flows continually. A charge is applied to the ink to charge the pigment particles which are electrostatically ejected onto the printing media, the majority of the solvent remains on the plate. The printer has no nozzle plate and as the ink flows continually over the plate the dispersant requirements of the ink are less stringent than those required in conventional inkjet printing. Succinimides are amongst the many classes of compound suggested as particle charging agents but there is no suggestion that succinimides may act as dispersants for the particles.
According to the present invention, there is provided an ink jet printer ink composition having a viscosity no greater than 35 m Pa.s comprising a dispersion of particulate pigment dye in a diluent and containing a dispersing agent from the pigment dye characterised in that the diluent comprises more than 50% by volume of white oil and the composition includes at least one dispersing agent comprising at least one compound having both an at least C36 aliphatic group and at least one xcex1, xcex2-di-carboxylic acid imide moiety, said at least one dispersing agent being present in an amount of at least 20% by weight of the pigment dye, wherein the pigment dye is a non-carbon black colorant characterised as a pigment dye in The Color Index.
The white oils suitable for use in the invention will contain less than 5% aromatics by weight. Preferably, the white oil is free or substantially free of aromatic components; that is to say it contains not more than 1% aromatics. Technical grade white oils are suitable and medicinal and food-grade white oils are preferred. The diluent may contain at least one other liquid component in addition to the white oil provided the white oil forms the major component of the diluent by volume and the diluent remains single phase. Thus, water, if present, must be in an amount which is miscible with or soluble in the other component or components of the diluent. Examples of other components that may be included in admixture with the white oil are other organic liquids provided that they do not interfere with colour, odour and/or stability. While the inclusion of such other organic liquids is not essential, it may be desirable in some cases to include at least one polar organic liquid, to enhance stability. Liquid fatty acid esters form a preferred class of such liquids. The amount of fatty acid ester that may be used will depend upon its nature and properties, eg. volatility, and upon the choice of other components in the formulation but amounts of up to 20% by volume have generally been found acceptable. Even higher amounts may be found useful is some formulations.
xe2x80x9cPigments dyesxe2x80x9d as that term is used herein means those non-carbon black colorants characterised as pigment dyes in The Colour Index